Implantable cardiac stimulation devices are well known in the art. Such devices apply electrical stimulation pulses to one or more chambers of the heart. The energies of the applied stimulation pulses are selected so as to be above the pacing energy stimulation threshold of the respective heart chamber to cause the cardiac tissue of that chamber to depolarize. Depolarization of the cardiac tissue of the respective chamber in turn causes the respective chamber heart muscle to contract. In this manner, the required pumping action of the heart is sustained.
It is therefore desirable to ascertain the pacing energy stimulation threshold of a heart chamber to be paced. A pacing energy may then be selected which is above the threshold to assure reliable pacing.
When a pacing pulse is effective in causing depolarization or contraction of the heart muscle, it is referred to as “capture” of the heart. Conversely, when a pacing pulse is ineffective in causing depolarization or contraction of the heart muscle, it is referred to as “lack of capture” of the heart.
An electrogram (EGM), as is also well known in the art, is an electrical signal representing the electrical activity of a heart muscle. The electrical manifestation of lack of capture in a heart muscle is typically a negative deflection in the electrogram baseline. This is referred to as polarization (POL). The electrical manifestation of capture in a heart muscle is typically an exaggerated biphasic deflection in the EGM. This is generally referred to as the evoked response plus polarization (ER+POL).
When a cardiac stimulation device performs a pacing energy stimulation threshold search or test, it applies a succession of test pacing pulses at a basic rate. The energy of each successive pacing pulse is reduced by a known amount and capture is verified following each pulse. Capture may be verified by detecting the evoked response.
Each stimulation includes a pair of pulses, a primary pulse and a subsequent backup pulse. The stimulation pulses of each pair are timed such that, if the primary pulse captures, the backup pulse will be delivered during the refractory period to provide a measure of polarization. The polarization waveform is subtracted from the evoked response plus polarization waveform to determine if capture occurred.
Sensing of evoked responses is therefore useful for capture verification and threshold assessment. Unfortunately, sensing of evoked responses is often difficult. Polarization after potentials tend to obscure the evoked responses when leads are used which have polarizing electrodes, such as electrodes formed of polarized platinum. Further, a number of leads, like active fixation screw-in leads continue to include electrodes formed of polarizing materials. Still further, leads that are retained for further use at device replacement tend be early generation leads having polarizing electrodes.
The present invention addresses the issues of sensing evoked responses. More specifically, as will be seen hereafter, the present invention provides for the automatic selection of the best evoked response sensing electrode configuration from among the most propitious evoked response sensing electrode configurations offered by a cardiac stimulation system.